There are few that spring to mind as great, Futurama’s numerous references and the excellent Bender’s Game, Gile’s excellent quote of “I used to be a highly respected Watcher and now I’m a wounded dwarf with the mystical strength of a doily” in the final episode of Buffy. There are some others that are less than great, specifically Big Bang Theories attempts to represent D&D, which are just painful to watch sometimes.
I think my favourite though has to go to Community, specifically the first D&D episode Advanced Dungeons and Dragons. This episode was brilliant, from the way that the characters were portrayed, to Pierce turning on the party, reading the adventure and then becoming the evil villain. The whole thing played out perfectly and even the props looked convincing enough that I actually looked it up to see if the adventure they play, The Caverns of Draconis, was real or not (it isn’t).
Still, that whole episode was awesome and it felt authentic, as if it had been written by someone who had played D&D and understood it and the inherent comedy in RPGs, rather than making jokes at the expense of the hobby based on assumptions. The writer, Dan Harmon, actually did play D&D and was interviewed on the WoTC Podcast about his experiences prior to the airing of the 2nd D&D episode.